SUMMARYAn ecological study was made of the freshwater distribution of Actinomycete virus. One component of this, the virus ofActinoplanes, is highly specific and its pattern of occurrence mirrors that of its host. The occurrence of Streptomyces virus is more difficult to explain in ecological terms, but this is attempted. Micromonospora virus was not obtained and the significance of this is discussed. The Actinoplanes virus is characterized at the fine‐structure level by a polyhedral‐shaped head with a tail bearing a contractile sheath, the latter feature being previously undescribed for actinoph